While you can paint good paintings with a bad brush, it's much harder to. So don't bother with bad brushes. They usually don't last long and are frustrating while they're around.
However, you don't need to start off with the most expensive brushes either. From medium range up, cost is not necessarily an indicator of quality. Once you've reached a certain level in brushes, it's more about the artist's skill than the quality of the brush. A good brush makes painting easier. It's not a magic wand!
Holding Your Watercolor Brush Correctly
Once you've picked out a watercolor brush, the next step is to hold it correctly. My youtube painting lesson below shows you how to use your brush as an extension of your hand!
Brush Control Exercises
Here are some useful brush exercises for improving brush control.
These are the easiest way to instantly improve your painting! Buy a pad of cheap paper, paint it through with these exercises and you'll have improved your brush control dramatically!